Abstract: A spark plug (100) includes a ground electrode (125), a firing electrode (115), an insulator core (105) having an insulator core nose (120), and an outer shell (110) surrounding the insulator core (105). The improvement includes the insulator core nose (120) including a high temperature glaze applied to an outer surface of the insulator core nose (120) and the high temperature glaze composition. The composition includes, by weight:(i) between about 72 and 84% SiO.sub.2 ;(ii) between about 10 and 20% Al.sub.2 O.sub.3 ;(iii) between about 2.0 and 2.5% K.sub.2 O;(iv) between about 0.50 and 0.65% Na.sub.2 O;(v) between about 3.0 and 5.5% CaO;(vi) between about 0.1 and 0.25% MgO; and(vii) the remainder impurities.

Abstract: A barium/lead-free ceramic glaze includes by weight:(i) between about 44 and 50% SiO.sub.2 ;(ii) between about 7 and 13% Al.sub.2 O.sub.3 ;(iii) between about 22 and 26% B.sub.2 O.sub.3 ;(iv) between about 3 and 9% CaO;(v) between about 1 and 3% SrO;(vi) up to about 2% ZnO;(vii) between about 2 and 4% K.sub.2 O;(viii) up to about 2% Li.sub.2 O; and(ix) between about 5 and 9% Na.sub.2 O.This barium/lead-free glaze can be effectually applied to a ceramic substrate at firing temperatures on the order of 1550 to 1650.degree. F. (843 to 899.degree. C.).